This invention relates generally to cryogenic coolers or refrigerators and, more particularly, to multistage cryogenic coolers employing pulse tubes.
Cryogenic coolers are typically used aboard spacecraft for cooling infrared detectors when temperatures below about 100 K are required, since simple radiators become very inefficient at these low temperatures. One type of cryogenic cooler that is frequently used is a closed-cycle expansion cooler which provides cooling through an alternating compression and expansion of a gas, with a consequent reduction of gas temperature. Typical cryogenic coolers of this type include Stirling, Vuilleumier, Gifford-McMahon, Joule-Thomson and pulse tube coolers. Pulse tube coolers are particularly attractive for space applications because they have no cold moving parts, which increases reliability and reduces vibration, and because they operate at comparatively low pressures with high efficiencies.
A single stage pulse tube cooler is generally capable of reaching temperatures of about 70-80 K., while still lower temperatures require some type of staging of the pulse tubes. U.S. Pat. No. 3,237,421 to Gifford and several papers, one by Ray Radebaugh entitled "Pulse Tube Refrigeration-A New Type of Cryocooler" and another by Yuan Zhou et al. entitled "Two-Stage Pulse Tube Refrigeration," disclose multistage pulse tube coolers in which all the heat from each successively lower-temperature pulse tube cooler is rejected to the preceding higher-temperature pulse tube cooler. Unfortunately, the large cooling load imposed on the higher-temperature pulse tube coolers considerably reduces the overall efficiency of the multistage cooler. Accordingly, there has been a need for an improved staging configuration for multistage pulse tube coolers. The present invention is directed to this end.